The field of biomedical optics is growing rapidly due to distinct advantages over other imaging modalities such as X-ray CT, MRI, nuclear medicine, or ultrasound (J. C. Hebden and D. T. Delpy. Diagnostic Imaging with Light, The British Journal of Radiology, 1997, 70, S206-S214; G. Freiherr. The Light Stuff: Optical Imaging in Medical Diagnostics, Medical Devices & Diagnostic Industry, 1998, 40-46). Compounds absorbing or emitting in the visible or NIR region of electromagnetic spectrum are potentially useful for tomographic imaging, endoscopic examination, photodynamic therapy, optoacoustic imaging, and sonofluourescene techniques. Furthermore, compounds absorbing or emitting in the appropriate visible region can be used to generate singlet oxygen and have been shown to be effective for photodynamic therapy of certain types of tumors.
Metal ions continue to play a major role in diagnostic and therapeutic medicine. For example, radionuclide metal complexes derived from both transition and lanthanide elements are being used extensively in diagnostic and therapeutic nuclear medicine procedures, paramagnetic complexes are being used extensively in magnetic resonance imaging procedures, and platinum complexes have long been used as cancer chemotherapeutic agents. Recently, metal complexes that absorb or emit in the visible or near-infrared (NIR) region have made a significant impact in the field of biomedical optics and have a great potential for photodiagnostic and phototherapeutic applications (J. N. Demas and B. A. DeGraff. Design and Applications of Highly Luminescent Transition Metal Complexes, Analytical Chemistry, 1991, 63, 829-837; M. P. Houline et al. Spectroscopic Characterization and Tissue Imaging Using Site-Selective Polyazacyclic Terbium (III) Chelates, Applied Spectroscopy, 1996, 50(10), 1221-1228; J. R. Lakowicz et al. Development of Long-Lifetime Metal-Ligand Probes for Biophysics and Cellular Imaging, Journal of Fluorescence, 1997, 7, 17-25; F. J. Steemers et al. Near-Infrared Luminescence of Yb.sup.3+, Nd.sup.3+, and Er.sup.3+ Azatriphenylene Complexes, Tetrahedron Letters, 1998, 39, 7583-7586; G. E. Keifer and D. J. Bornhop. Fluorescent Chelates as Visual Tissue Specific Imaging Agents, U.S. Pat. No. 5,922,867, 1999). Examples of suitable metal ions for optical applications include Cr(III), Os(II), Ru(II), Ni(II), Eu(III), Tb(III), Lu(III), Yb(III), Er(III), and Nd(III). Eu(III), and Tb(III) are particularly preferred because of favorable absorption and emission properties in visible and NIR regions.
The key requirements for design of novel metal complexes for optical diagnostic and therapeutic application are: (a) strong absorption and emission in the visible or NIR region; (b) high thermodynamic, kinetic, and photo stability; (c) low toxicity; (d) water solubility; and (e) conjugation capability for targeted delivery to particular tissues or organs. Free metal ions are generally quite toxic; they need to be administered in the form of complexes with complexing agents (ligands) in order to deliver them to specific organs and to alleviate toxicity. Electronic property, toxicity, stability, and tissue specificity are greatly affected by the nature of the complexing agents (ligands). Various physicochemical and pharmacokinetic factors have to be considered in order to render the metal complex safe and effective. Electronic requirements for enabling the metal ion (transition and lanthanide) to absorb or emit in the visible or NIR region are well established and essentially involve incorporation of metal into highly polarizable .pi.-electron rich, multidentate ligand systems. Energy transfer from aromatic donors (referred to as "antennae") to the lanthanide metal ion directly bounded to the donor group results in large increase in lanthanide fluorescence (S. I. Weissman, Journal of Chemical Physics, 1942, 10, 214; B. Alpha et al. Energy Transfer Luminescence of Europium (III) and Terbium (III) Cryptates of Macrobicyclic Polypyridine Ligands, Angewandte Chemie International Edition in English, 1987, 26(3), 266-267; J. B. Lamture et al. Luminescence Properties of Terbium (III) Complexes with 4-Substituted Dipiclolinic Acid Analogues, Inorganic Chemistry, 1995, 34, 864-869). In contrast, simple lanthanide metal salts or lanthanide ions coordinated to polyaminocarboxylate ligands wherein the aromatic donors are not directly attached exhibit very weak fluorescence in aqueous media (A. Abusaleh and C. F. Meares. Photochemistry and Photobiology, 1984, 39, 763-769). Long-wavelength fluorescence of transition metal complexes generally occurs via metal-to-ligand charge transfer (MCLT) interactions (Z. Murtaza and J. R. Lakowicz. Long-lifetime and Long-wavelength Osmium (II) Metal Complexes Containing Polypyridine Ligands. Excellent Red Fluorescent Dyes for Biophysics and for Sensors, SPIE, 1999, 3602, 309-315).
Toxicity of metal complexes is greatly affected by the nature of the ligands. Since in vivo release of free metal ions from the complex is a major cause of toxicity, thermodynamic and kinetic stability are critical requirements for the design of novel ligands. The thermodynamic stability constant indicates the affinity of totally unprotonated ligand for a metal ion. The conditional stability constant indicates the stability of the complex under physiological pH. Ion selectivity of the ligand toward the desired metal ion over other endogenous metal ions such as zinc, iron, magnesium, and calcium, determines the rate of release of the metal ion into the vascular or extracellular space. The released metal ion is capable of crossing the blood-brain barrier and thereby perturbing the neurophysiology. Therefore, in vivo reaction kinetics are also a major factor in the design of stable complexes and complexes with structural features that make in vivo transmettlation reactions proceed much slower than the biological clearance of the intact metal complexes would be predicted to have low toxicities (W. Cacheris et al., Magnetic Resonance Imaging, 1990, 8, 467; Oksendal et al., Journal of Magnetic Resonance Imaging, 1993, 3, 157). Thus, a need continues to exist for new and structurally diverse metal complexes for use as imaging, diagnostic, or therapeutic agents employing optical procedures.